Had the same problem. Read in another forum that Sierra might have disabled 3rd party adapters. I tested and can verify it is true, at least for now. I tried 2 3rd party mDP->VGA adapters that worked before Sierra, and MacBook screen flashed on plug-in but the monitor was not detected by the MBPR (late-2013 15in). Asked a friend for an Apple branded adapter and it worked right away. It is frustrating, hope Apple will be generous enough to allow 3rd party adapter for use again.

Thanks for your input; so I understand that the Apple branded mDP-VGA adapter works with Sierra?

Pity 3rd party ones don't, it's a deal breaker for me because as an educator I often have to connect to a projector, which almost all use a VGA connection; & might need to use a 3rd party connector.

I'm seriously thinking of staying with Yosemite even though Apple purposefully made the newer & better Safari upgrades (including a better zoom feature, & no the cc hacks to not work correctly) available only in Sierra.

Apple used to be "it just works", what a shame it's no longer the case.

I am experiencing this same issue, except it's a MacBook Pro Mid 2012, with a direct mini display port to display port cable. If the MacBook puts the display to sleep, then the external monitor will never come back on until I shut down the computer. A reboot doesn't fix it. This started with 10.12.1 was installed. Worked fine on 10.12.

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